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P codepowertrainMedium severity

P0669PCM/ECM/TCM Internal Temperature Sensor Circuit High

The control module's internal temperature sensor signal is stuck at the high end of its range, reading far hotter than plausible.

Definition

The PCM/ECM/TCM uses an internal temperature sensor to guard its driver circuits against overheating, and P0669 sets when that signal exceeds the maximum valid threshold — either a railed-high electrical signal or a reading hotter than the module could actually be. Because the sensor is inside the module, the usual cause is an internal circuit failure, but genuine module overheating from a bad mounting environment or failed cooling provision can also push the reading out of range. The MIL illuminates, and some applications derate outputs or alter fan strategy to protect the module; the code will cause an emissions-inspection failure while current. Distinguishing real overheating from a failed sensor is the core of the diagnosis.

Common causes

  • Internal failure of the module's temperature sensing circuit
  • Actual module overheating from exhaust proximity, a missing heat shield, or blocked cooling airflow
  • Poor module power or ground connections offsetting the analog reading
  • Moisture intrusion or corrosion inside the module
  • Under-hood temperatures elevated by an unrelated engine overheating condition

Common misdiagnoses

  • Replacing the ECT sensor for a perceived overheating complaint
  • Replacing the thermostat or cooling fans when the engine itself never overheated
  • Replacing the IAT sensor
  • Condemning the module without first checking its mounting environment and heat shielding

Troubleshooting steps

  1. 1.Compare the internal temperature PID to conditions

    Cold engine after an overnight soak should show the module near ambient; a reading pegged high (150 degrees C or scale maximum) at cold soak proves an electrical fault, not real heat.

  2. 2.Check the module's thermal environment

    Verify heat shields are installed, the module isn't touching hot components, and any cooling fins or airflow paths are clear of debris.

  3. 3.Rule out genuine engine overheating

    Confirm coolant temperature and fan operation are normal, since sustained under-hood heat can legitimately drive the module out of range.

  4. 4.Voltage-drop test module powers and grounds

    Repair any circuit dropping more than about 0.1 V under load before condemning the module.

  5. 5.Replace the module if the reading stays railed

    With a normal thermal environment and clean supplies, a persistent high-railed internal sensor means the module must be replaced and programmed.

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